The tyrosine kinase FRK/RAK participates in cytokine-induced islet cell cytotoxicity

The BCR::ABL1 tyrosine kinase inhibitors ponatinib and nilotinib differentially affect endothelial angiogenesis and signalling

BCR::ABL1 inhibitors, the treatment of choice for the majority of patients with chronic myeloid leukaemia (CML), can cause vascular side effects that vary between agents. The exact underlying mechanisms are still poorly understood, but the vascular endothelium has been proposed as a site of origin. The present study investigates the effects of three BCR::ABL1 inhibitors, ponatinib, nilotinib and imatinib, on angiogenesis and signalling in human endothelial cells in response to vascular endothelial growth factor (VEGF). The experiments were performed in endothelial cells isolated from human umbilical veins. After exposure to imatinib, ponatinib and nilotinib, the angiogenic capacity of endothelial cells was assessed in spheroid assays. VEGF-induced signalling pathways were examined in Western blotting experiments using different specific antibodies. RNAi technology was used to downregulate proteins of interest. Intracellular cGMP levels were measured by ELISA. Imatinib had no effect on endothelial function. Ponatinib inhibited VEGF-induced sprouting, while nilotinib increased spontaneous and VEGF-stimulated angiogenesis. These effects did not involve wild-type ABL1 or ABL2, as siRNA-mediated knockdown of these kinases did not affect angiogenesis and VEGF signalling. Consistent with their effects on sprouting, ponatinib and nilotinib affected angiogenic pathways in opposite directions. While ponatinib inhibited VEGF-induced signalling and cGMP formation, nilotinib activated angiogenic signalling, in particular phosphorylation of extracellular signal-regulated kinase 1/2 (Erk1/2). The latter occurred in an epidermal growth factor receptor (EGFR)-dependent manner possibly via suppressing Fyn-related kinase (FRK), a negative regulator of EGFR signalling. Both, pharmacological inhibition of Erk1/2 or EGFR suppressed nilotinib-induced angiogenic sprouting. These results support the notion that the vascular endothelium is a site of action of BCR::ABL1 inhibitors from which side effects may arise, and that the different vascular toxicity profiles of BCR::ABL1 inhibitors may be due to their different actions at the molecular level. In addition, the as yet unknown pro-angiogenic effect of nilotinib should be considered in the treatment of patients with comorbidities associated with pathological angiogenesis, such as ocular disease, arthritis or obesity.

Intracellular C3 protects β-cells from IL-1β-driven cytotoxicity via interaction with Fyn-related kinase

One of the hallmarks of type 1 but also type 2 diabetes is pancreatic islet inflammation, associated with altered pancreatic islet function and structure, if unresolved. IL-1β is a proinflammatory cytokine which detrimentally affects β-cell function. In the course of diabetes, complement components, including the central complement protein C3, are deregulated. Previously, we reported high C3 expression in human pancreatic islets, with upregulation after IL-1β treatment. In the current investigation, using primary human and rodent material and CRISPR/Cas9 gene-edited β-cells deficient in C3, or producing only cytosolic C3 from a noncanonical in-frame start codon, we report a protective effect of C3 against IL-1β-induced β-cell death, that is attributed to the cytosolic fraction of C3. Further investigation revealed that intracellular C3 alleviates IL-1β-induced β-cell death, by interaction with and inhibition of Fyn-related kinase (FRK), which is involved in the response of β-cells to cytokines. Furthermore, these data were supported by increased β-cell death in vivo in a β-cell-specific C3 knockout mouse. Our data indicate that a functional, cytoprotective association exists between FRK and cytosolic C3.

HIV-1 Vif suppresses antiviral immunity by targeting STING

HIV-1 infection-induced cGAS-STING-TBK1-IRF3 signaling activates innate immunity to produce type I interferon (IFN). The HIV-1 nonstructural protein viral infectivity factor (Vif) is essential in HIV-1 replication, as it degrades the host restriction factor APOBEC3G. However, whether and how it regulates the host immune response remains to be determined. In this study, we found that Vif inhibited the production of type I IFN to promote immune evasion. HIV-1 infection induced the activation of the host tyrosine kinase FRK, which subsequently phosphorylated the immunoreceptor tyrosine-based inhibitory motif (ITIM) of Vif and enhanced the interaction between Vif and the cellular tyrosine phosphatase SHP-1 to inhibit type I IFN. Mechanistically, the association of Vif with SHP-1 facilitated SHP-1 recruitment to STING and inhibited the K63-linked ubiquitination of STING at Lys337 by dephosphorylating STING at Tyr162. However, the FRK inhibitor D-65495 counteracted the phosphorylation of Vif to block the immune evasion of HIV-1 and antagonize infection. These findings reveal a previously unknown mechanism through which HIV-1 evades antiviral immunity via the ITIM-containing protein to inhibit the posttranslational modification of STING. These results provide a molecular basis for the development of new therapeutic strategies to treat HIV-1 infection.

SKAP2, a Candidate Gene for Type 1 Diabetes, Regulates β-Cell Apoptosis and Glycemic Control in Newly Diagnosed Patients

The single nucleotide polymorphism rs7804356 located in the Src kinase-associated phosphoprotein 2 (SKAP2) gene is associated with type 1 diabetes (T1D), suggesting SKAP2 as a causal candidate gene. The objective of the study was to investigate if SKAP2 has a functional role in the β-cells in relation to T1D. In a cohort of children with newly diagnosed T1D, rs7804356 predicted glycemic control and residual β-cell function during the 1st year after diagnosis. In INS-1E cells and rat and human islets, proinflammatory cytokines reduced the content of SKAP2. Functional studies revealed that knockdown of SKAP2 aggravated cytokine-induced apoptosis in INS-1E cells and primary rat β-cells, suggesting an antiapoptotic function of SKAP2. In support of this, overexpression of SKAP2 afforded protection against cytokine-induced apoptosis, which correlated with reduced nuclear content of S536-phosphorylated nuclear factor-κB (NF-κB) subunit p65, lower nitric oxide production, and diminished CHOP expression indicative of decreased endoplasmic reticulum stress. Knockdown of CHOP partially counteracted the increase in cytokine-induced apoptosis caused by SKAP2 knockdown. In conclusion, our results suggest that SKAP2 controls β-cell sensitivity to cytokines possibly by affecting the NF-κB-inducible nitric oxide synthase-endoplasmic reticulum stress pathway.

A high-throughput integrated microfluidics method enables tyrosine autophosphorylation discovery

Autophosphorylation of receptor and non-receptor tyrosine kinases is a common molecular switch with broad implications for pathogeneses and therapy of cancer and other human diseases. Technologies for large-scale discovery and analysis of autophosphorylation are limited by the inherent difficulty to distinguish between phosphorylation and autophosphorylation in vivo and by the complexity associated with functional assays of receptors kinases in vitro. Here, we report a method for the direct detection and analysis of tyrosine autophosphorylation using integrated microfluidics and freshly synthesized protein arrays. We demonstrate the efficacy of our platform in detecting autophosphorylation activity of soluble and transmembrane tyrosine kinases, and the dependency of in vitro autophosphorylation assays on membranes. Our method, Integrated Microfluidics for Autophosphorylation Discovery (IMAD), is high-throughput, requires low reaction volumes and can be applied in basic and translational research settings. To our knowledge, it is the first demonstration of posttranslational modification analysis of membrane protein arrays.

Understanding the cellular roles of Fyn-related kinase (FRK): implications in cancer biology

The non-receptor tyrosine kinase Fyn-related kinase (FRK) is a member of the BRK family kinases (BFKs) and is distantly related to the Src family kinases (SFKs). FRK was first discovered in 1993, and studies pursued thereafter attributed a potential tumour-suppressive function to the enzyme. In recent years, however, further functional characterization of the tyrosine kinase in diverse cancer types suggests that FRK may potentially play an oncogenic role as well. Specifically, while ectopic expression of FRK suppresses cell proliferation and migration in breast and brain cancers, knockdown or catalytic inhibition of FRK suppresses these cellular processes in pancreatic and liver cancer. Such functional paradox is therefore evidently exhibited in a tissue-specific context. This review sheds light on the recent developments emerged from investigations on FRK which include: (a) a review of the expression pattern of the protein in mammalian cells/tissues, (b) underlying genomic perturbations and (c) a mechanistic function of the enzyme across different cellular environments. Given its functional heterogeneity observed across different cancers, we also discuss the therapeutic significance of FRK.

A silent mutation made possible efficient production of active human Frk tyrosine kinase in Escherichia coli

Fyn-related kinase (Frk) was first identified using human breast cancer cells. It shares 51% identity with c-Src. Like all members of the Src family, Frk is thought to cause several cancers via dysregulations in signal transduction from cell-surface receptors. The excess activity of Frk on beta-cells has a crucial role in type-I diabetes. A silent mutation at Ile229 conferred a bacterial expression system on the kinase domains of Frk, which allowed for the quick expression and purification of one unphosphorylated and two mono-phosphorylated kinase domains. The C-terminal catalytic segment of the human Frk kinase conjugating hexahistidine purification tag (His-tag) was expressed in Escherichia coli. After first-step purification utilizing the His-tag, an anion-exchange chromatogram yielded three major peaks that had distinguishable phosphorylation characteristics as judged by Western blot analysis and measurement of kinase activity. This result of active protein production should promote drug discovery studies, including highthrough-put screening and structure-based drug design.

RAKing in AKT: a tumor suppressor function for the intracellular tyrosine kinase FRK

The Fyn related kinase FRK, originally called RAK, is a member of a small family of intracellular Src-related tyrosine kinases that includes PTK6 and Srms. These kinases share a conserved gene structure that is distinct from that of the Src family. Expression of FRK and PTK6 was originally identified in melanoma, breast cancer cells and normal intestinal epithelium, and both FRK and PTK6 have been implicated in the regulation of epithelial cell differentiation and apoptosis. Recently FRK was reported to phosphorylate the tumor suppressor PTEN (phosphatase and tensin homolog deleted from chromosome 10), a negative regulator of phosphatidylinositol 3 kinase (PI3K) signaling and AKT activation. FRK-mediated tyrosine phosphorylation of PTEN suppressed its association with NEDD4-1, an E3 ubiquitin ligase that may target it for polyubiquitination and proteosomal degradation. As a positive regulator of PTEN, FRK suppresses AKT signaling and inhibits breast cancer cell tumorgenicity in xenograft models. Both FRK and the related tyrosine kinase PTK6 appear to have multiple context-dependent functions, including the ability to regulate AKT. Although PTK6 negatively regulates AKT signaling in normal tissues in vivo, it may enhance AKT signaling in breast cancer cells. In contrast, FRK, which is expressed in the normal mammary gland but lost in some breast tumors, has tumor suppressor functions in mammary gland cells.

Increased Hsp70 expression attenuates cytokine-induced cell death in islets of Langerhans from Shb knockout mice

Type 1 diabetes may depend on cytokine-induced beta-cell death and therefore the current investigation was performed in order to elucidate this response in Shb-deficient islets. A combination of interleukin-1beta and interferon-gamma caused a diminished beta-cell death response in Shb null islets. Furthermore, the induction of an unfolded protein response (UPR) by adding cyclopiazonic acid did not increase cell death in Shb-deficient islets, despite simultaneous expression of UPR markers. The heat-shock protein Hsp70 was more efficiently induced in Shb knockout islets, providing an explanation for the decreased susceptibility of Shb-deficient islets to cytokines. It is concluded that islets deficient in the Shb protein are less susceptible to cytotoxic conditions, and that this partly depends on their increased ability to induce Hsp70 under such circumstances. Interference with Shb signaling may provide means to improve beta-cell viability under conditions of beta-cell stress.

A role of FRK in regulation of embryonal pancreatic beta cell formation

The fyn-related-kinase (FRK) is a non-receptor tyrosine kinase expressed in various tissues, and among them, is the islets of Langerhans. The role of FRK in pancreatic beta cells has been addressed by studies of knockout or FRK transgenic mice. These experiments have shown that FRK overexpression in beta cells leads to an increased susceptibility to the beta cell toxin streptozotocin and to cytotoxic cytokines, suggesting that FRK may participate in events leading to beta cell destruction. However, these mice also exhibit an increased relative beta cell volume and increased beta cell replication following partial pancreatectomy, suggesting a positive role for FRK in the regulation of beta cell number as well. To further assess the significance of FRK for beta cell replication, we studied the beta cell area and islet cell replication in FRK null mice. We currently observed that the FRK knockout mouse showed no difference in the insulin positive cell area or in the percentage of Ki67-stained proliferating islet cells at adulthood, when compared to wild-type control. In addition, adult FRK(-/-) mice performed normally when subjected to an intravenous glucose tolerance test. To elucidate whether FRK affects pancreatic beta cell number during embryogenesis and shortly after birth, pancreata were collected from FRK(-/-) mice at these stages. Histological analysis of insulin stained pancreatic sections showed that the insulin positive cell area in FRK(-/-) mice was reduced at embryonal day 15 and at birth to 31 and 70% of that of wild-type mice, respectively. FRK(-/-) pancreas weight on day 1 neonatally was similar to that of the control, indicating that the obtained results were not due to altered pancreatic growth. Taken together, these results show that FRK affects beta cell number during embryogenesis and early in life, but is probably redundant for beta cell number and function in adult animals under normal conditions.

Biological and biomaterial approaches for improved islet transplantation

Islet transplantation may be used to treat type I diabetes. Despite tremendous progress in islet isolation, culture, and preservation, the clinical use of this modality of treatment is limited due to post-transplantation challenges to the islets such as the failure to revascularize and immune destruction of the islet graft. In addition, the need for lifelong strong immunosuppressing agents restricts the use of this option to a limited subset of patients, which is further restricted by the unmet need for large numbers of islets. Inadequate islet supply issues are being addressed by regeneration therapy and xenotransplantation. Various strategies are being tried to prevent beta-cell death, including immunoisolation using semipermeable biocompatible polymeric capsules and induction of immune tolerance. Genetic modification of islets promises to complement all these strategies toward the success of islet transplantation. Furthermore, synergistic application of more than one strategy is required for improving the success of islet transplantation. This review will critically address various insights developed in each individual strategy and for multipronged approaches, which will be helpful in achieving better outcomes.

Gene silencing in the endocrine pancreas mediated by short-interfering RNA

OBJECTIVES

RNA interference as mediated by short-interfering RNA (siRNA) offers a nonviral means to silence genes in tissue; however, few data exist about gene therapy using siRNA in pancreas tissue. To determine if siRNA treatment could silence an endogenous gene in pancreatic islets, we developed a murine model using the endocrine pancreas.

METHODS

The insulin 2 (Ins2) gene was targeted with siRNA, and quantitative RT-PCR, fluorescent microscopy, and FACS were used to measure transcript levels and siRNA cellular uptake and transfection efficiency. Isolated pancreatic islets were transfected with siRNA in vitro using a liposomal delivery method in a dose titration (50-400 nM) or pooled from BALB/c mice having received siRNA (100 microg) via hydrodynamic tail vein injection.

RESULTS

The Ins2 transcript level was significantly reduced by 55% in vitro with FACS data showing a transfection efficiency over 45% with the 400 nM concentration. In vivo delivery of siRNA to pancreatic islets revealed a 33% reduction in Ins2 mRNA levels, although siRNA was able to be detected in 19% of isolated islet cells.

CONCLUSION

We have successfully used RNA interference to silence an endogenous tissue-specific gene (Ins2) in pancreatic islets when transfected in vitro or administered in vivo.

Clonal identification of multipotent precursors from adult mouse pancreas that generate neural and pancreatic lineages

The clonal isolation of putative adult pancreatic precursors has been an elusive goal of researchers seeking to develop cell replacement strategies for diabetes. We report the clonal identification of multipotent precursor cells from the adult mouse pancreas. The application of a serum-free, colony-forming assay to pancreatic cells enabled the identification of precursors from pancreatic islet and ductal populations. These cells proliferate in vitro to form clonal colonies that coexpress neural and pancreatic precursor markers. Upon differentiation, individual clonal colonies produce distinct populations of neurons and glial cells, pancreatic endocrine beta-, alpha- and delta-cells, and pancreatic exocrine and stellate cells. Moreover, the newly generated beta-like cells demonstrate glucose-dependent Ca(2+) responsiveness and insulin release. Pancreas colonies do not express markers of embryonic stem cells, nor genes suggestive of mesodermal or neural crest origins. These cells represent a previously unidentified adult intrinsic pancreatic precursor population and are a promising candidate for cell-based therapeutic strategies.